Process for production of a silica-supported alkali metal catalyst

ABSTRACT

A process for regenerating a silica-supported depleted alkali metal catalyst is described. The level of alkali metal on the depleted catalyst is at least 0.5 mol % and the silica support is a zero-gel. The process comprises the steps of contacting the silica supported depleted alkali metal catalyst with a solution of a salt of the alkali metal in a solvent system that has a polar organic solvent as the majority component. A re-impregnated catalyst prepared by the process of the invention any comprising a silicazero-gel support and a catalytic metal selected from an alkali metal in the range 0.5-5 mol % on the catalyst, wherein the surface area of the silica support is &lt;180 m 2 /g is also described. The invention is applicable to a process for preparing an ethylenically unsaturated acid or ester comprising contacting an alkanoic acid or ester of the formula R 1 —CH 2 —COOR 3 , with formaldehyde or a suitable source of formaldehyde.

The present invention relates to the production of alkali metalcatalysts on silica supports, particularly the regeneration of alkalimetal depleted catalysts.

Alkali metal catalysts on silica supports are known to be useful incatalysing various chemical processes. For instance, the alkali metalcaesium catalyses the aldol condensation of formaldehyde with alkylesters or acids to produce ethylenically unsaturated esters or acids, inparticular with methyl propionate to form methyl methacrylate (MMA) andmethacrylic acid (MA). However, in continuous industrial applications,the catalytic metal component and catalytic surface area are slowlydepleted over time causing consequential loss of catalyst activity. Itwould be advantageous, therefore, to be able to regenerate the catalyst.

WO99/52628 discloses caesium doped silica supported catalysts.WO99/52628 teaches that for the catalyst to be most effective thesurface area should be maintained. The document goes on to teach thegeneral process of impregnation mentioning various salts. No specificsolvent is given for caesium except water.

U.S. Pat. No. 4,990,662 discloses the use of metal salts during theprocess of impregnation. The impregnation of a support with Rb, Cs, Kand Na phosphates in aqueous solution by “incipient wetness” or the“pore filling technique” is described. Caesium carbonate is also usedand added during catalyst preparation by an unspecified method. Thispatent also describes a method of adding caesium to the catalyst as partof the vaporized feed to avoid depletion of the catalyst. This techniquehas the disadvantage of poor distribution of the caesium on the catalystand excessive coke formation at the front face of the catalyst bed.

U.S. Pat. No. 6,887,822 (PQ Corporation) describes production of asilica hydrogel supported alkali or alkaline earth metal catalyst byimpregnation of the hydrogel with an aqueous alkaline solution of thealkali or alkaline earth metal salt. However, the document also teachesthat the silica gel surface area is reduced at alkaline pH and elevatedtemperatures.

WO2009/003722 teaches the impregnation of the catalytic metal ontoinorganic oxide supports such as silica using an aqueous acid bath.

Surprisingly, a process has been found which restores catalyst activityto original levels without surface area treatment or damage to thecatalyst support.

According to the present invention there is provided a process forregenerating a silica-supported depleted alkali metal catalyst whereinthe level of alkali metal on the depleted catalyst is at least 0.5 mol%, and wherein the silica support is a zero-gel comprising the step of:—contacting said silica supported depleted alkali metal catalyst with asolution of a salt of the alkali metal in a solvent system that has apolar organic solvent as the majority component.

The process of the invention is particularly suitable for regenerationof a used silica-supported catalyst. Such catalysts will typically havea reduced surface area. For instance, the depleted and subsequentlyregenerated catalyst may have a surface area of <180 m²/g⁻¹, moretypically <150 m² g⁻¹. The surface area may be measured by well knownmethods, a preferred method being a standard BET nitrogen absorptionmethod as is well known in the art. Preferably, the bulk of the surfacearea of the silica is present in pores of diameter in the range 5-150nm. Preferably, the bulk of the pore volume of the silica is provided bypores of diameter in the range 5-150 nm. By the “bulk” of its porevolume or surface area is provided by pores of diameter in the range5-150 nm we mean that at least 50% of the pore volume or surface area isprovided by pores of this diameter and more preferably at least 70%.

In addition, the depleted alkali metal catalyst may additionally includea second or further metal selected from the group consisting ofzirconium, titanium, hafnium, aluminium, boron, and magnesium ormixtures thereof, preferably, zirconium, titanium, hafnium and aluminiumor mixtures thereof, most preferably, hafnium and zirconium or mixturesthereof.

Suitable alkali metals may be selected form lithium, sodium, potassium,rubidium and caesium, preferably, potassium, rubidium and caesium.Caesium is preferred. The salt of the alkali metal may be selected fromthe group consisting of acetate, propionate, carbonate, hydrogencarbonate, nitrate and hydroxide.

Surprisingly, it has been found that strongly alkaline salts, forexample alkali metal hydroxides such as caesium hydroxide may be used tore-impregnate the catalyst. This is surprising because it was understoodfrom U.S. Pat. No. 6,887,822 that exposing the catalyst support tostrongly alkaline salts would lead to hydrothermal ageing of the supportwith consequential damage to the catalyst and loss of surface area. Inaddition, exposing the catalyst to alkaline salts could lead todissolution of the silica.

It has been found to be advantageous to use an impregnation solventsystem that has a polar organic solvent as the majority component to actas carrier for the alkali metal salt in the impregnation. This solventsystem advantageously reduces heat generation, which can cause catalystbead cracking, and also reduces the risk of silica dissolution at highpH. Surprisingly, this is also contrary to the prior art teaching. U.S.Pat. No. 6,887,822 teaches that using an alcoholic solution of caesiumfor re-impregnation on a zero gel causes a high amount (76%) of beadcracking. In the current invention, it is found that doing the same on adepleted catalyst causes no such problems.

A preferred polar organic solvent is an alcohol such as a C₁-C₄ alkanol,especially, methanol. This polar organic solvent can be used alone asthe solvent system or mixed with an aliphatic ester, and/or with water.The aliphatic ester may be a C₁-C₆ alkyl C₁-C₆ alkanoate, more typicallya C₁-C₄ alkyl C₁-C₄ alkanoate, most typically, methyl propionate.Particularly suitable systems are provided wherein the polar organicsolvent is methanol and the aliphatic ester is methyl propionate such asthe azeotropic mixture thereof or wherein the polar organic solvent ismethanol. In either case, the solvent system may take up progressivelymore water as the impregnation progresses mainly due to water beingalready present on the catalyst to be treated but also due to theintroduction of water in an aqueous source solution of the alkali metalsalt prior to its addition to the polar organic solvent and also due toa small amount that is liberated from the reaction with the support. Ina series of batch reactions, it will be appreciated that the solventsystem will gradually be enriched in water as new catalyst batches areimpregnated and as alkali metal salt is added to replenish the solventsystem. The preferred solvent system commences with methanol without theuse of significant levels of aliphatic ester. Typically, methanol isused in conjunction with a caesium salt, more typically caesiumhydroxide. The use of such a combination causes water to beprogressively added and taken up into the solvent system duringimpregnation as explained above.

Preferably, the solution of the alkali metal salt in the solvent systemhas a starting pH between 8 and 13, more preferably, the solution of thealkali metal salt in the solvent system has a starting pH between 12 and13.

As mentioned above, a preferred salt is the hydroxide and the preferredpolar organic solvent is methanol.

A suitable concentration for the alkali metal in the solvent system atthe start of the impregnation is between 6×10⁻³ and 0.6 mol·dm⁻³ alkalimetal in the solution, more typically, between 18×10⁻³ and 0.18 mol·dm⁻³alkali metal in the solution, most typically, 30×10⁻³ and 0.12 mol·dm⁻³alkali metal in the solution.

Typically, the contacting step duration is sufficient to equilibrate thecatalyst support with the solution. Equilibration may be determined byno significant change in the alkali metal levels in the solutionresulting from further contact with the support. By significant changeis meant changes in the concentration of—5% or more, more typically,— 1%or more. Typically, equilibration may be undertaken in a few hours.

According to a second aspect of the present invention there is provideda re-impregnated catalyst prepared by the process of the first aspect ofthe invention optionally including any of the preferred or optionalfeatures thereof comprising a silica zero-gel support and a catalyticmetal selected from an alkali metal in the range 0.5-5 mol % on thecatalyst, wherein the surface area of the silica support is <180 m²/g.

In one embodiment, the catalyst contains between 0.5 and 2.0 wt % of thesecond metal. A particularly preferred second metal is zirconium. Thesecond metal improves the catalyst crush resistance as described in U.S.Pat. No. 6,887,822.

In the case where it is desired to impregnate a silica-supportedcatalyst with caesium using methanol as the solvent, any methanolsoluble caesium salt can be used, such as the carbonate, hydrogencarbonate, acetate, nitrate or propionate. The adsorption of caesium isfound to proceed most efficiently at high pH ˜13 and to reduce withfalling pH necessitating the use of a greater concentration of caesiumsalt in solution. Accordingly, the adsorption of caesium proceeds mostefficiently when a strongly basic caesium salt such as caesium hydroxideis used.

Surprisingly, the presence of water in the impregnation solution has noeffect on the caesium uptake efficiency. In this regard, the presence ofwater has been tested up to 44 wt % and found to have no appreciableeffect.

Water may typically be present in the impregnation solution at up to 40%by weight, more typically, up to 30% by weight in solution, mosttypically, up to 20% by weight.

In addition, significant levels of silica dissolution are avoided whenlow levels of alkali metal salt <2 wt % in solution are used.

Surprisingly, a used catalyst that has a depleted surface area of <180m²/g, more usually, <150 m²/g, and that has been regenerated by addingmore alkali metal as defined herein performs similarly in terms of % MMA& MAA yield and % MMA & MAA selectivity to a freshly prepared catalystof high surface area (>250 m²/g) containing the same amount of alkalimetal. This represents a considerable improvement in performance overthat achieved before catalyst regeneration.

Typically, the level of alkali metal on the depleted catalyst prior toimpregnation is at least 0.5 mol %, more typically, at least 1.0 mol %.Upper levels for the alkali metal on the depleted catalyst prior toimpregnation will depend on the reaction for which the catalyst has beenused. The level will be a depleted level for that reaction. Typically,the alkali metal will be present at a level of 0.5-3.0 mol %, moretypically, 1-3.0 mol % on the depleted catalyst.

Alternatively, the wt % of alkali metal may be at least 1 wt % or moretypically, at least 2 wt % on the depleted catalyst. Typically, thealkali metal is present in the range 1 to 6 wt % on the depletedcatalyst, more typically, 2-6 wt %, especially 4-6 wt %. These amountswould apply to all alkali metals but especially caesium.

Typically, the level of alkali metal in the catalyst after carrying outthe process of the invention is in the range from 1-5 mol % on thecatalyst, more typically, 2-4 mol %, most typically, 2.5-4 mol % on thecatalyst.

Alternatively, the re-impregnated catalyst may have a wt % of alkalimetal in the range 1 to 10 wt % on the catalyst, more typically 4 to 8wt %, most typically, 5-8 wt %. These amounts would apply to all alkalimetals but especially caesium.

The increase in alkali metal on the catalyst following the process ofthe invention is typically in the range 0.25 to 2.0 mol % on thecatalyst, more typically, 0.75 mol % to 1.5 mol %, most typically, 0.9to 1.4 mol %.

Alternatively, the typical increase in alkali metal is between 0.5 and 4wt % on the catalyst, more typically, between 1.5 and 3.5 wt %, mosttypically, between 2 and 3 wt %. These amounts would apply to all alkalimetals but especially caesium.

According to a third aspect of the present invention there is provided aprocess for preparing an ethylenically unsaturated acid or estercomprising contacting an alkanoic acid or ester of the formulaR¹—CH₂—COOR³, with formaldehyde or a suitable source of formaldehyde offormula I as defined below:

where R⁵ is methyl and R⁶ is H;

X is O;

n is 1;

and m is 1;

in the presence of a catalyst according to the second aspect of thepresent invention, and optionally in the presence of an alkanol; whereinR¹ is hydrogen or an alkyl group with 1 to 12, more preferably, 1 to 8,most preferably, 1 to 4 carbon atoms and R³ may also be independently,hydrogen or an alkyl group with 1 to 12, more preferably, 1 to 8, mostpreferably, 1 to 4 carbon atoms.

Preferably, the ethylenically unsaturated acid or ester is selected frommethacrylic acid, acrylic acid, methyl methacrylate, ethyl acrylate orbutyl acrylate, more preferably, it is an ethylenically unsaturatedester, most preferably, methyl methacrylate. Accordingly, the preferredester or acid of formula R¹—CH₂—COOR³ is methyl propionate or propionicacid respectively and the preferred alkanol is therefore methanol.However, it will be appreciated that in the production of otherethylenically unsaturated acids or esters, the preferred alkanols oracids will be different.

Accordingly, one particular process for which the re-impregnatedcatalysts of the present invention have been found to be particularlyadvantageous and/or from which the depleted catalysts may be obtained isthe condensation of formaldehyde with methyl propionate in the presenceof methanol to produce MMA.

In the case of production of MMA, the re-impregnated catalyst ispreferably contacted with a mixture comprising formaldehyde, methanoland methyl propionate.

Preferably, the mixture comprising formaldehyde, methanol and methylpropionate contains less than 5% water by weight. More preferably, themixture comprising formaldehyde, methanol and methyl propionate containsless than 1% water by weight. Most preferably, the mixture comprisingformaldehyde, methanol and methyl propionate contains 0.1 to 0.5% waterby weight.

The term “alkyl” when used herein, means unless otherwise indicated, C₁to C₁₀, preferably, C₁ to C₄ alkyl, and alkyl includes methyl, ethyl,propyl, butyl, pentyl, hexyl, and heptyl groups and is most preferablymethyl.

In the third aspect of the present invention, the alkanoic acid or esterthereof and formaldehyde can be fed, independently or after priormixing, to the reactor containing the catalyst at molar ratios of acidor ester to formaldehyde of from 20:1 to 1:20 and at a temperature of250-400° C. with a residence time of 1-100 seconds and at a pressure of1-10 bara.

In the first aspect of the present invention, the re-impregnation may becarried out under any suitable conditions, for example, ambienttemperatures and pressures. Suitable temperatures are 0-100° C., moretypically 5-60° C., most typically, 10-50° C. Suitable pressures for thereaction are 1-10 bara.

Typically, the catalyst is in the form of a fixed bed during contactwith the alkali metal solution which are thereby passed therethrough.

Suitable flow rates for the alkali metal solution in contact with thecatalyst are 0.1 to 10 bed volumes/hr, more typically 0.2 to 2 bedvolumes/hr, most typically 0.4 to 1 bed volumes/hr.

By bed volume is meant the amount equivalent to the bulk volume of thebed of catalyst to be treated.

By majority component of a solvent system is meant a component thatmakes up at least 50% by volume of the solvent system, more suitably, atleast, 60%, most suitably, 70% or more. The majority component may makeup 95% or more, for example, 99% or more, or approximately 100% of thesolvent system by volume. If the majority component does not make up100% by volume of the solvent system, the balance of the solvent systemmay be made up of one or more co-solvents.

By solvent system herein is meant a single solvent or a solvent togetherwith one or more co-solvents. By single solvent is meant more than 98%by volume, more typically, more than 99% by volume of the solventsystem. Accordingly, by co-solvent is meant a solvent that makes up atleast 1% by volume of the solvent system, more typically, at least 2% byvolume.

By zero-gel is meant a dried support typically, wherein >90% of thewater has been removed from the hydrogel, more typically, >95%, mosttypically, >99%. A zerogel may contain up to 6% water by weight, moreusually, 3-5% by weight.

By mol % on the catalyst herein is meant mol % relative to moles ofsilica (SiO₂) in the catalyst. It is therefore assumed for the purposeof calculation that silica has a molecular weight equivalent to SiO₂rather than that of a silica network. This more accurately reflects thenature of the catalyst. For example, 1 wt % caesium would equate to 0.45mol % caesium in the catalyst, assuming molecular weights of 132.9 and60.1 respectively.

Unless indicated to the contrary, amounts of alkali metal or alkalimetal catalyst relate to the alkali metal ion and not the salt.

Levels of alkali metal on the catalyst whether mol % or wt % may bedetermined by appropriate sampling and taking an average of suchsamples. Typically, 5-10 samples of a particular catalyst batch would betaken and alkali metal levels determined and averaged, for example byXRF analysis.

The catalysts will normally be used and re-impregnated in the form of afixed bed and so it is desirable that the catalyst is formed into shapedunits, e.g. spheres, granules, pellets, aggregates, or extrudates,typically having maximum and minimum dimensions in the range 1 to 10 mm.The catalysts are also effective in other forms, e.g. powders or smallbeads.

It will be appreciated that the process of the invention is a liquidphase impregnation process.

The invention will now be described by way of example only withreference to the following examples and drawings in which:—

FIG. 1 is a schematic view of apparatus for carrying out the process ofthe invention.

Referring to FIG. 1, a 2″ glass chromatography column 2 has a taperedlower end 12 connecting the column 2 to an inlet tube 14. The inlet tube14 is connected to Gilson pump 8 via a T connection 20 and pump outlettube 16. The T connection 20 includes a drain tube 18 which may be usedto drain fluid from the column and a switch (not shown) for directingfluid flow from the pump 8 or to the drain tube 18 as required. A pumpinlet tube 22 connects the Gilson pump to the base outlet of thereservoir flask 6. A recycling conduit 10 connects the top of the column2 to the top inlet of the reservoir flask 6 to allow fluid pumped upthrough the bed to be recycled to the reservoir 6. In the embodimentshown, the column 2 contains 400 g of catalyst beads supported on a frit4 located across the base of the column. The use of the apparatus willbe described more particularly hereafter and with reference to theexamples.

EXAMPLES Caesium Regeneration of an Exhausted Catalyst

Used Catalyst

In all examples, samples from the same batch of used and depletedcaesium on silica/zirconia catalyst beads (5.05 wt % Cs, 0.86 wt % Zr,130 m²/g) were used. This catalyst, when fresh, had 6.7 wt % Cs on itwith 0.86 wt % Zr and had a surface area of 327 m²/g. pH measurements,where quoted, were made by adding an equal volume of water to a sampleof solution and observing the colour change in a pH paper immersed init.

Example 1

A 1.2 wt % solution of caesium in methanol was prepared using caesiumcarbonate (Cs₂CO₃, 99% Reagent Plus from Aldrich) and dry methanol(<1000 ppm water). 400 g of used catalyst beads were placed in a 2″glass chromatography column 2 with a glass frit 4 at the bottom. 1000 mlof caesium solution in methanol was charged to a 2 litre flask 6(catalyst: solution ratio φ=0.4 kg/litre) and pumped up-flow at 25ml/min through the catalyst bed from the bottom of the column by aGilson pump 8. Solution that had passed through the bed was returned tothe flask via a recycling conduit 10 in the column 2 above the level ofthe catalyst. XRF analysis (Oxford Instruments X-Supreme8000) was usedto measure the caesium content in solution for the starting feed andperiodic samples of the return flow from the column. The solution wasrecirculated in this way until XRF analysis showed that a steady-statecaesium concentration in solution had been reached, which occurred after2 hrs, when it was measured at 0.55 wt % (54.1% uptake from solution)

The methanol solution was then drained from the bed under gravity for 1hour and the catalyst beads dried in situ by passing a current of drynitrogen up-flow through the bed at −200 ml/min overnight. 650 ml of theoriginal solution was recovered after draining and the dried catalystwas found to have 6.72 wt % Cs as measured by XRF. Of the 1.67 wt %increase in caesium, 1.28 wt % was calculated to have come from uptakefrom the solution and 0.39 wt % from evaporation of methanol solutionremaining in the pores.

Example 2

The remaining caesium in methanol solution from Example 1 (0.55 wt % Cs,650 ml) was made up to 1000 ml with fresh methanol and extra caesiumcarbonate added to increase the concentration of caesium in solution to1.38 wt %. A fresh 400 g of used catalyst was then regenerated using thesame method as Example 1 to yield, after drying, a catalyst with 6.78 wt% caesium on it. The remaining solution contained 0.79 wt % caesium byXRF (42.4% uptake from solution).

Example 3

The remaining caesium in methanol solution from Example 2 (0.79 wt % Cs,650 ml) was made up to 1000 ml with fresh methanol and extra caesiumcarbonate added to increase the concentration of caesium in solution to1.40 wt %. A fresh 400 g batch of used catalyst was then regeneratedusing the same method as Example 1 to yield, after drying, a catalystwith 6.68 wt % caesium on it. The remaining solution contained 0.93 wt %caesium by XRF (33.5% uptake from solution).

Example 4

The remaining caesium in methanol solution from Example 4 (0.93 wt % Cs,650 ml) was made up to 1000 ml with fresh methanol and extra caesiumcarbonate added to increase the concentration of caesium in solution to1.402 wt %. A fresh 400 g batch of used catalyst was then regeneratedusing the same method as Example 1 to yield, after drying, a catalystwith 6.73 wt % caesium on it. The remaining solution contained 0.87 wt %caesium by XRF (36.8% uptake from solution).

Example 5

The remaining caesium in methanol solution from Example 4 (0.87 wt % Cs,650 ml) was made up to 1000 ml with fresh methanol and extra caesiumcarbonate added to increase the concentration of caesium in solution to1.361 wt %. A fresh 400 g batch of used catalyst was then regeneratedusing the same method as Example 1 to yield, after drying, a catalystwith 6.62 wt % caesium on it. The remaining solution contained 0.91 wt %caesium by XRF (33.2% uptake from solution).

Example 6

The remaining caesium in methanol solution from Example 5 (0.91 wt % Cs,650 ml) was made up to 1000 ml with fresh methanol and extra caesiumcarbonate added to increase the concentration of caesium in solution to1.191 wt %. A fresh 400 g batch of used catalyst was then regeneratedusing the same method as Example 1 to yield, after drying, a catalystwith 6.44 wt % caesium on it. The remaining solution contained 0.79 wt %caesium by XRF (33.6% uptake from solution).

Accordingly, re-cycling of drained equilibrium wash by replenishmentwith Cs₂CO₃/methanol results in a reducing uptake from solution, whichresults in greater initial concentrations of caesium being required toobtain the same uptake by the catalyst. (Examples 2-6).

Example 7

A sample of used and caesium depleted catalyst from the same batch (5.05wt % Cs, 0.86 wt % Zr, 130 m²/g) was regenerated using the method ofExample 1, but using 300 g of catalyst and 1500 ml of methanol solution(catalyst: solution ratio φ=0.2 kg/litre) containing 0.5 wt % caesiuminitially at 100 ml/min. After recirculation for 2 hrs 1200 ml of theoriginal solution was recovered, which contained 0.24 wt % caesium byXRF (51.9% uptake from solution). The regenerated catalyst had, afterdrying, 6.25 wt % caesium on it as measured by XRF. Of the 1.2 wt %increase in caesium, 1.04 wt % was calculated to have come from uptakefrom the solution and 0.16 wt % from evaporation of methanol solutionremaining in the pores.

Accordingly, halving of the catalyst-to-solution ratio, φ, did notsignificantly affect the proportion of caesium adsorbed during the wash(˜50% uptake was observed, in both Examples 1 and 7 with differentinitial caesium concentrations). (Example 7).

The decrease in excess caesium obtained on the beads after draining anddrying is consistent with a decrease in the strength of the equilibriumsolution. (Examples 1 to 7).

TABLE 1 Results of Repeated Catalyst Regenerations using CaesiumCarbonate Wt % Cs Increase on Final % Wt % Cs Catalyst from Wt % CsCaesium Increase on Evaporation Measured on taken Catalyst from ofSolution Catalyst by up from Example solution in Pores XRF Solution 11.28 0.39 6.72 54.1% 2 1.16 0.57 6.78 42.4% 3 0.93 0.70 6.68 33.5% 41.02 0.66 6.73 36.8% 5 0.89 0.68 6.62 33.2% 6 0.79 0.60 6.44 33.6% 71.04 0.16 6.25 51.9%

Example 8

1000 ml of a 0.785 wt % solution of caesium in methanol was preparedusing 7.84 g CsOH.H₂O as the Cs source with methanol. Karl Fishermeasurement of the initial water concentration showed there was 0.284 wt% water present and an approximate pH value of 13.0.

A sample of used catalyst was regenerated using the method of Example 1.After recirculation for 2 hrs the solution contained 0.078 wt % caesiumby XRF (90% uptake from solution) and 1.145 wt % water and had a pH of8.5. The regenerated catalyst had, after drying, 6.71 wt % caesium on itas measured by XRF. Of the 1.66 wt % increase in caesium, 1.4 wt % wascalculated to have been taken up from solution and 0.26 wt % fromevaporation of methanol solution remaining in the pores.

Accordingly, the use of a high pH wash solution of caesium gives agreater uptake efficiency even from a lower concentration of caesium.(Example 8).

Example 9

1000 ml of a 0.787 wt % solution of caesium in methanol/water (90:10)was prepared using 7.97 g CsOH.H₂O as the Cs source in a 10 wt % waterin methanol solvent mixture. Karl Fisher measurement of the initialwater concentration showed there was 10.63 wt % water and a pHmeasurement of 13.0.

A sample of used catalyst was regenerated using the method of Example 1.After recirculation for 2 hrs the solution contained 0.104 wt % caesiumby XRF (86.8% uptake from solution) and 10.77 wt % water and had a pH of8.5. The regenerated catalyst had, after drying, 6.8 wt % caesium on itas measured by XRF. Of the 1.75 wt % increase in caesium, 1.35 wt % wascalculated to have been taken up from solution and 0.4 wt % fromevaporation of methanol solution remaining in the pores.

Example 10

1000 ml of a 1.56 wt % solution of caesium in water/methanol (water wasadded to assist dissolution) was prepared using caesium bicarbonateCsHCO₃ as the Cs source. Karl Fisher measurement of the initial waterconcentration showed there was 11.58 wt % water and a pH measurement of9.0.

A sample of used catalyst was regenerated using the method of Example 1.After recirculation for 2 hrs the solution contained 1.104 wt % caesiumby XRF (29.2% uptake from solution) and 13.12 wt % water and had a pH of7.5. The regenerated catalyst had, after drying, 7.07 wt % caesium on itas measured by XRF. Of the 2.02 wt % increase in caesium, 0.91 wt % wascalculated to have been taken up from solution and 1.11 wt % fromevaporation of methanol solution remaining in the pores.

Example 11

1000 ml of a 1.18 wt % solution of caesium in methanol was preparedusing caesium carbonate (Cs₂CO₃, 99% Reagent Plus from Aldrich) andmethanol. Karl Fisher measurement of the initial water concentrationshowed there was 0.167 wt % water and a pH measurement of 12.5.

A sample of used catalyst was regenerated using the method of Example 1.After recirculation for 2 hrs the solution contained 0.54 wt % caesiumby XRF (54.2% uptake from solution) and 1.177 wt % water and had a pH of9.0. The regenerated catalyst had, after drying, 7.05 wt % caesium on itas measured by XRF. Of the 2.0 wt % increase in caesium, 1.27 wt % wascalculated to have been taken up from solution and 0.73 wt % fromevaporation of methanol solution remaining in the pores.

TABLE 2 Analyses of End Solution after Catalyst Regeneration Final wt %Cs in Final Si solution wt % H₂O in Initial Final (ppm) in End bysolution by pH of pH of Solution by Example XRF-I KF Solution SolutionICP-OES 8 - CsOH 0.078 1.145 13.0 8.5 1.0 9 - CsOH 0.104 10.77 13.0 8.5— 10 - 1.104 13.12 9.0 7.5 2.6 CsHCO₃ 11 - Cs₂CO₃ 0.540 1.177 12.5 9.030

Hence, the presence of larger amounts of water in the starting washsolution does not significantly affect the efficiency of caesium uptake.(Example 9).

Partially neutralised caesium salts of lower starting pH exhibit a muchlower caesium uptake efficiency than those with higher pH. (Examples 9,10 and 11).

High pH solutions of caesium salts in the presence of water do not causesignificant amounts of silica dissolution at the concentrations used(<0.1 wt % Si).

Catalyst Testing

Regenerated catalysts from Examples 7 to 11 were tested in a lab scalereactor alongside a standard fresh catalyst and the originalun-regenerated catalyst. 3 g of each catalyst was heated to 350° C. in atube reactor and pre-conditioned overnight with a vaporized feed streamcomprising 59.4 wt % methyl propionate, 29.7 wt % methanol, 3.9 wt %formaldehyde and 6.9 wt % water supplied from a pre-vaporizer fed by aGilson pump at 0.032 ml/min. The reactor exit vapor flow was condensedand sampled at five different feed pump rates to obtain conversions atdifferent vapor contact times with the catalyst. The condensed liquidproducts and the liquid feed were analysed by a Shimadzu 2010 GasChromatograph with a DB1701 column. The composition of the samples werethen determined from the gas chromatography data and the % yield and %selectivity to methacrylate (MMA+MAA) calculated. The results are shownin Table 3.

The testing of catalysts produced in Examples 7 to 11 show that similarresults to fresh catalyst are obtained in terms of % MMA & MAA yield and% MMA & MAA selectivity in the catalysis of formaldehyde condensationwith methyl propionate to produce MMA. They also show a considerableimprovement when compared with the performance of the caesium depleted,used catalyst before regeneration.

TABLE 3 Results of Regenerated Catalyst Testing % MMA + % MMA + ExampleMAA Yield MAA Selectivity Wt % Cs SA (m²/g) Fresh 10 96.68 6.3 327Before 8* 95.50 5.05 130 Regeneration 7 10 96.80 6.25 — 8 10 96.24 6.71120.8 9 10 96.02 6.8 112.4 10 10 95.95 7.07 114.9 11 10 96.36 7.05 112.4*Maximum yield obtained at any contact time

Attention is directed to all papers and documents which are filedconcurrently with or previous to this specification in connection withthis application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A process for regenerating a silica-supported depleted alkali metalcatalyst wherein a level of alkali metal on the depleted catalyst is atleast 0.5 mol %, and wherein the silica support is a zero-gel comprisingthe step of: contacting said silica supported depleted alkali metalcatalyst with a solution of a salt of the alkali metal in a solventsystem that has a polar organic solvent as the majority component. 2.The process according to claim 1, wherein a surface area of the catalystis <180 m²/g.
 3. The process according to claim 1, wherein a bulk of asurface area of the silica is present in pores having a diameter rangingfrom 5 nm to 150 nm.
 4. The process according to claim 3, wherein a bulkof pore volume of the silica is provided by pores having diametersranging from 5 nm to 150 nm.
 5. The process according to claim 1,wherein the alkali metal is selected from lithium, sodium, potassium,rubidium, and caesium.
 6. The process according to claim 1, wherein thesalt of the alkali metal is selected from the group consisting ofacetate, propionate, carbonate, hydrogen carbonate, nitrate, andhydroxide.
 7. The process according to claim 1, wherein the depletedalkali metal catalyst comprises a second metal selected from the groupconsisting of zirconium, titanium, hafnium, aluminium, boron andmagnesium, and mixtures thereof.
 8. The process according to claim 1,wherein the polar organic solvent is an alcohol.
 9. The processaccording to claim 1, wherein the solution of the alkali metal salt inthe solvent system has a starting pH between 8 and
 13. 10. The processaccording to claim 1, wherein water is present in the solution at up to40% by weight.
 11. The process according to claim 1, wherein the levelof alkali metal salt in solution is <2 wt %.
 12. The process accordingto claim 1, wherein an increase in alkali metal on the catalystfollowing the process is in the range 0.25 to 2.0 mol %.
 13. The processaccording to claim 1, wherein alkali metal concentration in the solventsystem to initially begin impregnation is between 6×10⁻³ and 0.6mol·dm⁻³ alkali metal in the solution.
 14. The process according toclaim 1, wherein the process is carried out at a temperature of 0-100°C.
 15. The process according to claim 1, wherein the process is carriedout at a pressure of 1-10 bara.
 16. The process according to claim 1,wherein the catalyst is a fixed bed form during contact with the alkalimetal solution.
 17. The process according to claim 1, wherein flow ratesfor the alkali metal solution in contact with the catalyst are 0.1 to 10bed volumes/hr.
 18. The process according to claim 1, wherein thecatalyst is formed into shaped units having maximum and minimumdimensions in the range 1 to 10 mm.
 19. The process according to claim1, wherein the process is a liquid phase impregnation process.
 20. Are-impregnated catalyst prepared by the process of claim 1 comprising: asilica zero-gel support and a catalytic metal selected from an alkalimetal in the range 0.5-5 mol % on the catalyst, wherein: a surface areaof the silica support is <180 m²/g.
 21. A process for preparing anethylenically unsaturated acid or ester comprising: contacting analkanoic acid or ester of the formula R¹—CH₂—COOR³, with formaldehyde ora suitable source of formaldehyde of formula I as defined below:

where R⁵ is methyl and R⁶ is H; X is O; n is 1; and m is 1; in thepresence of a re-impregnated catalyst, and optionally in the presence ofan alkanol; wherein R¹ is hydrogen or an alkyl group with 1 to 12 carbonatoms and R³ is independently, hydrogen or an alkyl group with 1 to 12carbon atoms, and wherein the re-impregnated catalyst comprises a silicazero-gel support having a surface area of <180 m²/g, and a catalyticmetal selected from an alkali metal in the range 0.5-5 mol % on there-impregnated catalyst.
 22. A process according to claim 21, whereinthe ethylenically unsaturated acid or ester is selected from methacrylicacid, acrylic acid, methyl methacrylate, ethyl acrylate, or butylacrylate.
 23. The process according to claim 21, wherein the ester oracid of formula R¹—CH₂—COOR³ is methyl propionate or propionic acid. 24.The process according to claim 21, wherein the alcohol is methanol. 25.The process according to claim 21, wherein the process is conducted at amolar ratio of acid or ester to formaldehyde from 20:1 to 1:20, and at atemperature of 250-400° C. with a residence time of 1-100 seconds, andat a pressure of 1-10 bara.